In multicarrier transmission schemes, a frequency band is divided into multiple narrow frequency bands (subcarriers) and separate signals are transmitted using the subcarriers. For example, in orthogonal frequency division multiple access (OFDMA), subcarriers are arranged such that they become orthogonal to each other to improve frequency efficiency and to achieve high-speed, high-volume communications. OFDMA makes it possible to effectively reduce inter-subcarrier interference. This in turn makes it possible to concurrently transmit signals using subcarriers and to increase the symbol length. Also with OFDMA, it is possible to effectively reduce multipath interference by using a relatively long guard interval.
With multicarrier transmission schemes, however, fairly-high peak power is instantaneously necessary for transmission because signals mapped to subcarriers overlap each other in the time domain. In other words, with multicarrier transmission schemes, the peak-to-average power ratio (PAPR) may become fairly high. This is not preferable particularly for mobile terminals.
Generally, single-carrier transmission schemes have an advantage in terms of reducing the PAPR. Particularly, single-carrier transmission schemes such as single-carrier frequency division multiple access (SC-FDMA) and discrete Fourier transform (DFT) spread OFDM also make it possible to efficiently use a wide frequency band. In SC-FDMA, a transmission signal is Fourier-transformed and mapped to subcarriers, and the mapped signal is inverse-Fourier-transformed and wirelessly transmitted. At the receiving end, a received signal is Fourier-transformed, signal components mapped to the subcarriers are extracted, and transmission symbols are estimated. Such single-carrier transmission schemes are preferable in terms of efficiently using a frequency band while reducing the PAPR.
Meanwhile, with single-carrier transmission schemes where subcarriers with a relatively large bandwidth are used, multipath interference tends to occur. Multipath interference increases as the transmission rate increases. For example, multipath interference becomes particularly prominent when the data modulation level is high or a MIMO multiplexing scheme is used. Increase in multipath interference in turn reduces the detection accuracy of signals at the receiving end.
Let us assume that the number of transmitting antennas is N, the data modulation level is B (e.g., when 16 QAM is used, B=4), the expected number of multipaths is P, and a maximum likelihood detection (MLD) method is used for signal detection (for a QRM-MLD method, see, for example, K. J. Kim, et al., “Joint channel estimation and data detection algorithm for MIMO-OFDM systems”, Proc. 36th Asilomar Conference on Signals, Systems and Computers, November 2002). As described above, OFDMA makes it possible to effectively reduce inter-subcarrier interference and to sufficiently reduce multipath interference within a guard interval. Therefore, with OFDMA, the total number of symbol candidates that need to be examined at the receiving end is represented by the following formula:2N×B 
Meanwhile, with a single-carrier transmission scheme where multipath interference cannot be ignored, the total number of symbol candidates that need to be examined is represented by the following formula:2N×B×P 
Thus, with a single-carrier transmission scheme, the number of candidates increases exponentially according to the number of multipaths and as a result, the computational complexity for signal detection increases. This in turn makes it difficult to employ an MLD method, which provides high detection accuracy but require high computational complexity, together with a single-carrier MIMO transmission scheme. Signal detection methods such as a zero forcing (ZF) method and a minimum mean squared error (MMSE) method require low computational complexity, but may reduce the signal detection accuracy. To achieve desired signal quality (desired SINR) when the signal detection accuracy at the receiving end is low, it is necessary to increase the transmission power of signals. However, since one purpose of employing a single-carrier transmission scheme is to reduce the PAPR and thereby to save battery energy, it is not preferable to increase the transmission power.